Device for maintaining a flexible line under a substantially constant tensile stress

ABSTRACT

This device for maintaining substantially constant the tensile stress applied to a flexible line particularly adapted for equipping the drilling floating installations, comprises the combination of a stationary return member and a return member moveable with respect to one holding point of the flexible line, said line passing successively over said return members while forming at least a loop portion, the moveable return member being supported at the end of a pivotable arm actuated by jack means fed with pressurized fluid from an oil and air accumulator, the jack rod having, for each position of the moveable return member, a direction different from that of the travel path of said moveable return member.

United States Patent 1 Joubert et al.

11] 3,749,367 [451 July 31,1973

1 1 DEVICE FOR MAINTAINING A FLEXIBLE LINE UNDER A SUBSTANTIALLY CONSTANT TENSILE STRESS [75] Inventors: Philippe Joubert, Meudon-La-Foret;

Yvon Castel, Le Pecq, both of France [73] Assignee: Institut Francais du Petrole, Des

Carburants et Luhriilants, Rueil-Malmaison, France [22] Filed: Dec. 8, 1971 21] Appl. No.: 205,922

[30] 4 Foreign Application Priority Data Feb. 16, 1970 France 7045510 [52] U.S. Cl 254/189, 254/172, 254/175.7

[51] Int. Cl B6611 3/08 [58] Field of Search 254/175.5, 175.7, 254/109, 172

[56] References Cited UNITED STATES PATENTS 3,662,991 5/1972 Lakiza 212/3 2/1967 Seward 212/3 12/1970 Graham 254/1757 [57] ABSTRACT This device for maintaining substantially constant the tensile stress applied to a flexible line particularly adapted for equipping the drilling floating installations, comprises the combination of a stationary return member and a return member moveable with respect to one holding point of the flexible line, said line passing successively over said return members while forming at least a loop portion, the moveable return member being supported at the end of a pivotable arm actuated by jack means fed with pressurized fluid from an oil and air accumulator, the jack rod having, for each position of the moveable return member, a direction different from that of the travel path of said moveable return member.

2 Claims, 5 Drawing Figures PATENIED JUL 3 1 I875 SHEET 2 0F 5 I PATEMIEnJum I973 SHEET '4 0F 5 qdE DEVICE FOR MAINTAINING A FLEXIBLE LINE UNDER A SUBSTANTIALLY CONSTANT TENSILE STRESS This invention relates to a device for maintaining a flexible line under a substantially constant tensile stress.

This device is, in particular, adapted to maintain at a substantially constant value the tensile stress applied to a flexible line of given length, fastened at both ends to points which are displaceable with respect to each other. In fact the displacement of one of the holding points of the flexible line results in variations of the tensile stress applied to the flexible line, which may be consequently subjected to abnormally high tensile stresses, liable to result in the breakage thereof, or, on the contrary to tensile stresses which are insufficient for insuring a correct stretching of the flexible line between its two holding points.

By way of non-limitative example the device according to the invention is advantageously used in the equipment of a floating installation connected, through flexible lines, to a submerged installation lying on the water bottom. These flexible lines may consist, for example, of anchoring cables or mooring ropes which have to be maintained under a substantially constant tensile stress so as to prevent their breakage or in order to achieve an automatic anchoring of the floating installation in the manner described in French Pat. No. 1.525.882, in spite of the vertical motions or shifts of the floating installation, due to the swell.

The flexible lines, by this way, will also act as guiding lines connecting the floating installation to an immersed installation laying on the water bottom, which lines must always be sufficiently stretched in spite of the movements of the floating installation, so as to prevent that they get entangled and to insure that they provide for a satisfactory guiding of apparatus between the two installations.

These lines may also be used for handling, from the floating installation, an apparatus or a tool lying on the water bottom and on which it is desired to apply a constant load in spite of the amplitude of the vertical movements-of the floating installation, the load applied to the tool depending on the tensile stress exerted on the flexible line.

' For maintaining substantially constant the tensile stress exerted on the flexible line, fixed at both ends to fastening points displaceable with respect to each other, use is made at first of winches exerting a constant tensile stress. These winches, comprising a reversible hydraulic motor provide for the application of a substantially constant tension to a cable the relative displacement of the cable fastening points being compensated by a corresponding winding or unwinding of a cable portion about the winch.

This solution suffers from somedrawbacks. As a matter of fact, as it is well known in the art, the cable winding tension is very different from the cable unwinding tension, said difference being generally greater than percent of the tension exerted on the cable. Moverover, besides the high price of these winches, the alternate movements of one of the cable fastening points result in the alternate winding and unwinding of the same cable portion, which, by contact with the turnsalready wound on the winch, becomes quickly worn, due to the repeated frictions to which it is subjected; this may result in the breaking of the cable. In addition, the use of a hydraulic device for driving the winch requires an expensive maintenance and watching.

Other devices have been used which compensate for the vertical displacement of the floating installation due to the swell, without making use of winches of the type exerting a constant tension.

Such devices include a pulley over which the flexible line passes, said pulley being suspended from the end of a hydraulic jack fed with a pressurized fluid supplied by an oil and air accumulator, the jack being placed substantially in a vertical direction.

Such device overcomes the disadvantages of the winches of the constant tension type and provides for the maintenance in a stretched state of the cable connecting the surface installation to the sea bottom, .but it makes it impossible to maintain substantially constant the tension exerted on the cable.

In order to improve such devices, it was contemplated to combine with the jack of the above-described devices, one or more inclined jacks. By this way the tension exerted on the cables was kept substantially constant.

However these last-mentioned devices still suffer from a major drawback, when used on drilling platforms or ships.

.In fact, in these devices, the pulley over which passes the cable, is subjected to displacements with an amplitude of the same order of magnitude as the vertical movement of the floating installation, which makes necessary the use of large-sized hydraulic jacks and requires the building, on the floating installation, of derricks for the only purpose of supporting such devices.

In addition to a large bulk, always detrimental in the case of a floating installation, the performaces of such devices which are limited by design, are further difficult to modify in view of compensating for the amplitudes of vertical movements greater than the maximum values for which it was initially designed.

--,It is .amainobject of this invention to provide a device for maintaining substantially constant the tension applied to a flexible line, fastened at both ends thereof to two holding points displaceable with respect to each other, so as to overcome the inconveniences of the prior devices.

Another object of the invention is to provide a simple device for regulating the tension applied to a flexible line,said device being optically associated to a member for .pulling on said flexible line such as, for example, a capstan, a pulling caterpillar and the like.

The device according to the invention, for maintaining .under asubstantially constant tension a flexible line of given length, provided, at both ends thereof, with two holding points displaceable with respect to each other, comprises a 'moveable return member such as, return pulley means, on which passes the flexible line, said member being supported by at least one jack, exerting the tension force and .fed with pressurized fluid from at least one accumulator, said jack being articulated, at a vflrst end thereof, to a point which is stationary with respect to one of the flexible line holding points. This .device is remarkable by the combination, with said mcveable return'member, of a returnmember in a fixed position with respect to said holding point of the flexible line, said flexible line passing successively over said fixed and moveable return members, thus .forming at least a loop ,portion, and by the provision of guiding means determining the travel path of said moveable return member, said jack co-operating with said guiding means for displacing said moveable member, the direction of the jack rod being, for each position of said moveable return member, different from the direction of the travel path of said moveable member.

According to a preferred embodiment, the device for maintaining under a substantially constant tension a flexible line of given length provided at its ends with two holding points displaceable with respect to each other, comprises the combination of a return member, stationary with respect to one of the holding points and a return member moveable along a predetermined travel path, the flexible line successively passing over said return members, thus forming at least a loop portion, two hydraulic jacks fed with pressurized fluid from an accumulator, said jacks being articulated at points which are stationary with respect to said holding point of the flexible line and supporting said moveable return member, the direction of the rods of the jacks being, for each position of said moveable return member, different from the direction of the travel path of said moveable member, said jacks being placed symmetrically with respect to the designed travel path of said moveable member so as to determine said travel path.

Other advantages will become apparent from the following description of non-limitative embodiments of the invention given with reference to the illustrative accompanying drawings wherein:

FIG. 1 diagrammatically shows a first embodiment of the device of the invention, equipping a floating installation,

FIG. 2 shows the various parameters which characterize the device of FIG. 1,

FIG. 3 is a diagram showing the curve of the tension variations as a function of the vertical amplitude of movement of the ship in the particular case of embodiment of FIG. 1,

FIG. 4 illustrates a second embodiment of the device according to the invention, and

FIG. 5 shows the preferred embodiment of the invention.

FIG. 1 illustrates in a non-limitative manner a first embodiment of the invention for maintaining constant the tension applied to a flexible line connecting a floatinginstallation, such as a ship, to a submerged apparatus lying on the water bottom.

On this figure the floating installation or ship has the reference N. On the water bottom 1, is placed an apparatus 2, eg a mooring, an underwater well head, a tool etc.

The ship N is connected to the apparatus 2 through a cable 3, fixed at one end to apparatus 2 and at the other end, on the ship N, to a device 4, such as a winch, a capstan, a pulling caterpillar etc.

In order that the tension T,,, applied to the cable, be kept constant, it is necessary to compensate for the vertical displacements of the ship,'due to the effect of the swell. For this purpose the line 3 is wound on the fixed return member 5, shown on the figure as a pulley 5a rotatable about its axis which is fastened to the end of a bracket 6 solid with the ship N. The cable 3 is then wound on a moveable return member 7, shown as a pulley rotatable about its axis, secured to the end of a pivotable arm 8 articulated at point 9 on the ship N.

The cable 3 is thereafter wound on a second pulley 5h having the same axis as pulley 5a, before being wound on a storage apparatus 4, e.g a winch, maintained in a locked position, i.e so that the line 3 cannot be unwound.

For sake of clarity the pulleys 5a and 5b have been shown in the drawings with different diameters but they may also have the same diameter. Moreover, only a single pulley 7 and two pulleys 5a and 5b are shown whereas the number of pulleys n and n forming respectively each return member 5 and 7 may be greater than I, n and n being either equal to each other or different by one unit according to the location selected for the apparatus 4 on the ship deck.

A hydraulic jack 10 is articulated at one of its ends 1 1 onto the ship, its other end being articulated at point 12 onto the arm 8. The hydraulic fluid feeding said jack is maintained under pressure by means of an oil and air accumulator 13.

When, under the action of the swell, the ship comes closer to the water bottom 1, the tension of cable 3 tends to decrease.

The hydraulic fluid pressure acts on the rod 14 of the jack 10 which drives the articulated arm 8 so that the pulley 7 takes the position 7, thus compensating for the tension decrease in cable 3.

When, under the action of the swell, the ship moves away from the water bottom 1, the tension of cable 3 increases and acts on 'the articulated arm 8 against the antagonistic action of the jack 10. The pulley 7 thus takes the position 7" at which the tension of the cable is substantially equal to its initial value.

It can be readily understood from the above that, in response to a tension change in the cable, due to the vertical movements of the ship, the articulated jack l0 acts on the arm 8, thereby displacing the pulley 8 and changing accordingly the length of the loop formed by cable 3 between the return members 5 and 7, so as to maintain substantially constant the tension applied to cable 3.

By this way, the inconveniences of the winches exerting a constant tensile force, for which the same cable portion is alternatively wound and unwound and comes in friction with the already wound turns on the winch reel, are overcome.

As it is apparent in FIG. 1, the only function of apparatus 4 is to exert a traction determining the cable length between the ship N and the submerged installation 2 and there can also be used conventional apparatuses forming the usual equipment of a floating installation such as a winch, a capstan, a pulling caterpillar etc.

The pulleys forming the return members are advantageously sheave pulleys on which the sliding of the cable is practically nullified, e.g by lining the sheaves of these pulleys with a deformable material such as rubber or a suitable plastic material.

These pulleys may also exhibit a low friction coefficient so as to reduce as much as possible the wear of cable 3.

The articulated arm 8 may be formed by devices already present on the ship and mounted for example on a mast-crane so as to leave free the ship deck.

FIG. 2 diagrammatically shows the various parameters used for determining the size of the device corresponding to the embodiment shown in FIG. 1.

The articulated arm 8 has a length R. This arm rotates about its pivot 9 by a maximum angle 2 B so as to compensate for the vertical movements of the ship, and drives the pulley 7 along the path V which, in the present particular case, is an arc of circle having a radius R.

r will be selected of the lowest possible value which is compatible with the material strength requirements for the manufacture of the jack. However, with low values of r, there can be used a jack whose piston stroke is short but it becomes necessary to make use of accumulators having a higher pressure P,, or, while increasing the piston section, to make use of a greater number of The straight line Oy indicates the mean position of 5 accumulators. the articulated arm 8 and will be used hereinafter as "fiaiiaaiiythsvaneor r will beso selected as to reference axis together with the straight Ox, make possible the use of jacks and accumulators of the perpendicular to 0y at point 0 corresponding to the commercial type. pivot point 9 in the plane of the path V. 2 The vatiationof Aiis iri inverse ratio to the dis The jack 10 is placed in the plane of the path V. It 10 tance a, i.e. the jack must be selected as long as possiis articulated at one of its ends 11 onto the ship N and, ble 7 p v at its other end 12, onto the arm 8. The distance be- 3. The variation of AT is proportional to the differtween the pivot points 9 and 12 is called r, whereas the ence (b-r). in fact,'in the position shown in solid line coordinates of the pivot point 11 in the'plane (Ox, 0y 1 5 in FIG. 2, the piston rod beingin its external position, are a and b. the pressure in accumulator 13 will be at its minimum The respective positions of pivots 11 and 12 are sovalue and, in order to achieve an actual compensation selected that the jack l0 always is inclined by an angle of the tension variations, the active component of the a with respect to the travel path portion of pulley 7. force exerted by the jack on the arm 8, i.e. the compoln the plane (Ox, 0y the corrdinates of the pulley. nent perpendicular to the articulated arm 8 will have to 5 are called 0 and d. be as great as possible and, accordingly, the angle a as In the following there will be determined the influsmall as possible. I V ence of the various parameters on the functional charlri practical conditions, those skilled in the art will seacteristics of the device of this invention, in order to be lect the values of the parameters in accordance with able to adapt it to particular conditions of use. the type of use'contemplated.

By experience, the amplitude A of the vertical movein a particular embodiment of the device according ments of the ship, to be compensated, is known. The to the invention and of its conditions of use, the charactension T which is desired to apply to cable 3 and to teristics were as follows: maintain substantially constant is also known and the percent variation AT of the maximum permissible ten- H sion is set by choice, eg at 5 percent. The maximum maximum amplitude of vertical movement of angular displacementZfi and the length R of the articufiaz g on the flexible line) 3 metric lated arm 8 are set so as to avoid the congestion of the 5 (section f the jack piston) omnmz ship deck. a L6 in Knowing the tension T,,,'one skilled in the art can easb 1.4 m ily determine the required characteristics of the oil and r 0.5 m air accumulator l3, i.e. its initial volume V, and its in- R 2.6 m 1 flation pressure P,,. v 1 I 2B= 34 (correspondingto a maximum travel of pul- Knowing the amplitude A, the maximum angular dis- 4o ley 7 of 1.5 m) v placement 23 and the length R, it is possible to detern (number of return pulleys of the return member mine the member of pulleys 7 and 5 to be used. As a =-l matter of fact a greater amplitude of vertical movement T, (Tension 'on the cable in the external position of of the ship can becompensated by using a longer articthe jack rod) 0.95 T,,, thus corresponding to-a ulated arm or a greater merber n of pulleys 7. permissible tension variation AT of 5 The mere use of a different member of pulleys will The values of the tensions exerted on the cable, in 10 accordingly change the regulation capacity of the dedifferent positions of the articulated arm, are given in vice. the following table: 1 I

Arm position I 2 3 4 s 6 7 s 9 l0 Tension on the cables in metric tons a 3.024 3.049 3.072 3.088 3.094 3.082 3.045 2.971 2.850

By calculation it can be established that the position A The position 1 corresponds to the external position of pulleys 5, with respect to the articulated arm 8', is not of the jack rod,.the position 10 to its internal position critical. In fact, all the parameters being constant, the shown in semi-dashed line in FIG. 2. variations in the tension on the cable, calculated as a The curve of FIG. 3 shows, in percentage of the value function of the above-defined parameters c and d, are of 3 metric tons, the tension variation in the cable, as always kept lower than 10 percent of the variation AT a function of the position of the articulated arm, or of observed during the operation of the device. the amplitude A of the vertical movement of the ship.

A thorough study of the influence of parameters r, a As it is apparent on said curve, the tension variationand 17 gives the following results: AT in the lineis lower than 3 percent during 95 percent 1. The variation of AT is proportional to r", in which of the displacement of the articulated arm as compared k has a value comprised between 2 and 3. Accordingly to an initially accepted variation of 5 percent.

Moreover, for the amplitudes of vertical movement which correspond to periods of operation under normal conditions, i.e amplitudes of 1.5 to 2 m, the tension variation in the cable is about 1 percent, i.e much lower than the accepted variation,

The embodiment illustrated in the preceding figures is given by way of non-limitative example.

Thus, the mean position of the articulated arm 8, shown by the axis OY in FIG. 2 may be vertical or inclined. The jack means may consist of a single jack placed in the plane of the travel path of pulley 7, but said jack may also be replaced by one or more jacks which may be inclined with respect to the plane of the pulley travel path. Particular safety devices may also be provided for the case where the amplitude of the ship vertical movement would exceed the forecast maximum value.

These safety devices may consist of members for detecting the position of the articulated arm 8, e.g. electric contacts which, when the articulated arm passes beyond one of the forecast limit positions, actuate the device 4 in order to increase or reduce the length of the flexible line 4 between the surface installation N and the apparatus 2.

Other changes may be made to the device without departing from the scope of the invention. For example the jack 10 may be replaced by any other elastic device such as a spring system or any other motive device adapted to displace the moveable member in correlation with the vertical displacements of the ship.

A second embodiment of the invention is illustrated in FIG. 4, in which reference numeral 1 indicates the sea bottom.N is a floating installation such as a drilling ship having a flexible line 3 fastened at one of its ends to an apparatus 2 lying on the sea bottom and being held, at its other end, by a device 4, such as a winch, a capstan, a pulling caterpillar etc.

At the end of a bracket, diagrammatically shown with reference 6 and solid with the ship N, there is placed a stationary return member 5, eg a pully rotatably mounted about its axis.

A moveable return member 7, eg a pulley rotatable about its axis, may be displaced along a guiding path, diagrammtically shown with reference 15, which determines the travel path of said moveable return member 7.

The flexible line 3 successively passes over said re turn members while forming at least a loop portion, as shown in FIG. 4.

A hydraulic jack [0 is articulated at point 11 onto the floating installation, whereas the rod 14 of this jack is articulated at point 12 onto the moveable return member. This jack is maintained under pressure by means of an oil and air accumulator 13.

The pivot ll-of tligjack is so located that the force exerted by said jackon the moveable return member 7 always has a component in the direction of the travel path of the moveable member 7, i.e the direction of the jack, for each position of the moveable return member 7, is different from the travel path of said moveable member as determined by the guiding member 15.

The operation of this device is simple and similar to that of the device shown in FIG. 1.

When the floating installation comes closer to the sea bottom 1, the tension in cable 3 is decreased and, as a result of the action of the jack 10, the moveable return member 7 comes to the position shown on the drawing in semi-dashed line, thus compensating for the vertical movement of the ship N and maintaining substantially constant the tensile stress exerted on the flexibleline 3.

When the floating installation moves away from the sea bottom, the tension of cable 3 tends to increase and displaces, against the antagonistic action of the jack 10, the moveable return member 7 to the position shown in solid line in FIG. 4, thereby compensating for the vertical movement of the ship and maintaining substantially constant the tension exerted on the cable 3.

Modifications can be made to the device without departing from the spirit and the scope of the invention. Thus the guiding path 15 can be rectilinear as shown' in the accompanying drawing, or incurved, according to the selection made by those skilled in the art in view of the contemplated particular conditions of use,

There can also be used one or more jacks optionally, but not necessarily, placed in the plane of the travel path of the moveable member 7.

The return members 5 and 7 may respectively consist of n and n being either identical numbers equal to or greater than 1 or numbers differing from each other by 1, according to the location of the device 4 on the floating installation, a higher member of pulleys providing for the compensation of larger vertical movements of the ship, without increasing the displacement of the moveable member 7.

In the case where the vertical movements of the ship have a greater magnitude than that which can be compensated by the device, there can be used safety members such as electric contracts, placed at the ends of the guiding path 8, for controlling the winding or the unwinding of cable 3 about the winch 4.

FIG. 5 illustrates the preferred embodiment of the device of this invention, the advantage of this embodiment resulting from the novel arrangement of the various elements, whereby can be achieved a compact device, having substantially the shape of a parallelipiped, and a small size, which makes easier its transportation.

In this figure, the device is shown in its operating position. It comprises a frame 16 carrying the stationary return member consisting of three pulleys 5a, 5b and 5c rotatably mounted on parallel axes, respectively 17a, 17b and 170, solid with the frame 16, only the axes 17a and being apparent in the drawing.

The pulleys 5a and 5b are placed in a common first plane, whereas the pulley 5c is placed in a different plane, parallel to said first plane.

The moveable return member consists of two parallel pulleys 7a and 7b freely rotatable about a common axis 18. The axis 18 is perpendicular to axes 17a, 17b and 170 and the diameter of pulleys 7a and 7b is chosen equal to thespacing between the planes containing respectively the pulleys 5a, 5b and 5c.

The axis 18 is solid with a pivot axis 19 about which are articulated the ends of the rods 20a and 20b of the two jacks 21a and 21b respectively. The jacks are articulated at their other ends about axes 22a and 22b respectively, which axes have a stationary position with respect to'the frame 16, during the operation of the device.

These two jacks are placed symmetrically with respect to a vertical plane containing the axes 17a and 19 and insure the displacement of the pulleys 7a and 7b along the travel path indicated by reference V. The jacks are connected to a group of oil and air accumulators, not shown in FIG. 5.

The flexible line 3, stored on the reel of a winch 4, passes successively on the pulleys 5a, 70, 5c, 7b and 5b, as shown in the figure.

The operation of the device is similar to that described with reference to FIG. 4.

When .the tension of the flexible line 3 tends to decrease, the rods a and 20b of the jacks 21a and 21b, displace the moveable pulleys 7a and 7b along the travel path V, in the direction shown by the arrow.

When the tension of the flexible line 3 tends to increase, it results in a displacement of the moveable pulleys 7a and 7b in a direction opposite to that of the arrow, against the antagonistic action of the jacks, thereby regulating the tension of the flexible line 3.

The pivot axes 22a and 22b of the jacks 21a and 21b are secured connecting rods to links 23a and 23b respectively rotatable about axis 170. In the operating position of the device, a transverse rod 24, only one part of which is apparent on the drawing, connects the pivot axes 22a and 22b, preventing any rotation of the links 23a and 23b about axis17c and thus locking the axes 22a and 22b with respect to the frame 16.

During transporation, the bulk of the device is reduced by removing the transverse rod 24, rotating the links in the direction of the arrows and housing the maximum portion of the rods in the jack cylinders, which brings the moveable pulleys closer to the stationary pulleys.

in order to make easier the winding or the unwindin of the flexible line about the winch reel when the latter is placed in close vicinity to the tension regulating device, provision is made for a possible displacement of the pulleys 5a and 5b with respect to the winch. These pulleys are interconnected with junction rods 25 and 26, the pulley axes being slidablein slots 27' and 28 respectively.

.A jack 29, solid with the frame 16 acts by its rod, on an axis 30 connecting the junction rods 25 and 26 and slidable in a slot 31 of the frame. These slots may, as illustrated in the drawings, consist of beams having an U-shaped cross-section. I

The jack 29 is fed from a hydraulic circuit, not shown, and actuates pulleys 5a and 5b, so that the portion 30 of the flexible line 3 be placed with a favorable angle for its winding on the winch reel.

Of course, the operation of the jack 29 may be automatically controlled by means of any known device when the amplitude of the vertical movement of the ship is greater than the regulation capacity of the device according to the invention, in view, according to the case, of the winding or the unwinding of a portion of the flexible line 2 on the winch reel.

In the embodiment illustrated in FIG. 5, the two pulleys 5a and 5b are displaceable by the action of the jack 29, so that the winch can be placed either on the same side as pulley 5a or on the side of pulley 5b, according to the location of the different apparatuses on the ship,

but it would be still within the scope of the invention to actuate by means of the jack 29, only one of these pulleys.

What we claim is:

l. A device for maintaining substantially constant the tensile stress exerted on a flexible line of given length, having its ends fastened to two holding points moveable with respect to each other, comprising the combination of a return member stationary with respect to one of said holding points and a return member moveable along a determined travel path the flexible line passing successively over said return members while forming at least one loop portion, two hydraulic jacks fed from an accumulator of pressuized fluid, said jacks being articulated at stationary points with moveable to said holding point of the flexible line and supporting said moveable return member, the direction of the jack rods being, for each position of said moveable return member, different from that of the travel path of said moveable return member, said jacks being placed symmetrically with respect to the designed travel path of said moveable return member so as todetermine said travel path, said stationary, return member consisting of three pulleys rotatable about parallel axes, two of said pulleys being in a common plane and the third one in a plane parallel to said common plane, said moveabe return member comprising two pulleys freely rotatable about a common axis which is perpendicular to the axes of the pulleys-forming the stationary return member, the diameter of the pulleysof the moveable return member being substantially equal to the distance between the planes containingthe pulleys of the stationary return member said jacks being articulated at one of their ends, about an axis solid with the axes of the moveable pulleys, and at their other ends about axes supported by links, rotatable about the axis of said third pulley of the stationary return member, said links having a first position, corresponding to the operating condition of the device, in which they are interconnected through a transverse rod locking said links in position with respect to said holding point of the flexible line, and a second position, adapted for the transportation of the device, inv which said links are brought closer to each other so as to reduce the bulk of the device as much as possible.

2. A device according to claim 1, associated to a storage reel for the flexible line, said reel being one of said holding points of the flexible line, wherein at least one of said two pulleys in the common plane of the stationary return member is displaceable under the action of a hydraulic jack, stationary with respect to said holding point, and whose rod acts on the axis of said pulley in response to the winding or unwinding of the flexible 

1. A device for maintaining substantially constant the tensile stress exerted on a flexible line of given length, having its ends fastened to two holding points moveable with respect to each other, comprising the combination of a return member stationary with respect to one of said holding points and a return member moveable along a determined travel path the flexible line passing successively over said return members while forming at least one loop portion, two hydraulic jacks fed from an accumulator of pressuized fluid, said jacks being articulated at stationary points with moveable to said holding point of the flexible line and supporting said moveable return member, the direction of the jack rods being, for each position of said moveable return member, different from that of the travel path of said moveable return member, said jacks being placed symmetrically with respect to the designed travel path of said moveable return member so as to determine said travel path, said stationary return member consisting of three pulleys rotatable about parallel axes, two of said pulleys being in a common plane and the third one in a plane parallel to said common plane, said moveab e return member comprising two pulleys freely rotatable about a common axis which is perpendicular to the axes of the pulleys forming the stationary return member, the diameter of the pulleys of the moveable return member being substantially equal to the distance between the planes containing the pulleys of the stationary return member said jacks being articulated at one of their ends, about an axis solid with the axes of the moveable pulleys, and at their other ends about axes supported by links, rotatable about the axis of said third pulley of the stationary return member, said links having a first position, corresponding to the operating condition of the device, in which they are interconnected through a transverse rod locking said links in position with respect to said holding point of the flexible line, and a second position, adapted for the transportation of the device, in which said links are brought closer to each other so as to reduce the bulk of the device as much as possible.
 2. A device according to claim 1, associated to a storage reel for the flexible line, said reel being one of said holding points of the flexible line, wherein at least one of said two pulleys in the common plane of the stationary return member is displaceable under the action of a hydraulic jack, stationary with respect to said holding point, and whose rod acts on the axis of said pulley in response to the winding or unwinding of the flexible line about the storing reel. 